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Microsoft officially ended support for Windows XP on April 8, 2014. That means the company is no longer patching newly discovered security vulnerabilities in the operating system, and people who continue to use it are opening themselves up to security risks.

However, according to NetMarketShare, more than a quarter of all PCs (27.69 percent) were still running Windows XP in March of this year.

Why would people continue using a twelve-year-old operating system that would put them at risk?

No doubt, many are home users who simply aren't very technology savvy and/or may not have the desire or the money to upgrade to a newer version of Windows. Some probably have older, underpowered PCs that can't run Windows 7 or 8. And others have specific software—often custom business applications—that only runs on Windows XP.

Fortunately, the open source community has free operating systems that meet the needs of users in all of these situations. This month we've put together a list of 50 different applications that can replace Windows XP. It's organized into several different categories. Those that are easiest for beginners to use come first, followed by lightweight operating systems that can run on old hardware, then operating systems that are particularly tailored for business users and open source operating systems that aren't based on Linux. The list ends with a few applications that aren't complete operating systems but do allow users to run their existing XP software from Linux.

As always, if you have another Linux distribution or other application that you think should have been on this list, feel free to write a note in the comments section below.

Before we get to the list itself, here's a some quick background for Windows XP users who aren't familiar with Linux or open source software. Linux is an operating system that anyone can use free of charge. In addition, anyone can see the source code for Linux and modify it however they like. Because anyone can tweak it, it comes in thousands of different versions, which are known as "distributions." Different Linux distributions use different interfaces or "desktops," which determine how the operating system looks on the screen. Unlike Windows, Linux distributions generally come with lots of free applications already built in, so users don't have to pay extra for office productivity software, security software, games or other applications.

Beginner-Friendly Linux Distributions

Many people consider Linux Mint to be among the most intuitive operating systems for Windows XP users. It supports several different desktop interfaces, including Cinnamon, which users can configure to look and feel a lot like XP.

Very easy to use, Ubuntu is likely the most widely used Linux distribution in the world. The desktop version offers speed, security, thousands of built-in applications and compatibility with most peripherals.

Built specifically to attract former Windows users, Ubuntu-based Zorin is probably the Linux distribution that's the most similar to Windows. It includes a unique "Look Changer" that switches the desktop to look like Windows 7, XP, Vista, Ubuntu Unity, Mac OS X or GNOME 2, and it includes WINE and PlayOnLinux to allow users to keep using their Windows software.

Also similar to Windows, Robolinux promises to allow users to run all their Windows XP and 7 software without making themselves vulnerable to malware. It also includes more than 30,000 open source applications.

Formerly known as YLMF, the interface for StartOS looks an awful lot like Windows XP. It's managed by a group of Chinese developers, so the website is in Chinese. However, English versions of the OS are available.

According to the Pinguy website, "PinguyOS is very much designed for people who are new to the Linux world; many people coming from both a Windows or a Mac background will find plenty of familiar features along with some new ones that aren’t available in either Windows or Mac." It's based on Ubuntu and uses the Gnome-Shell desktop.

Popular with new Linux users, MEPIS aims at providing a Linux distribution that's very stable and very easy to use. It comes with hundreds of applications preinstalled and you can easily dual-boot it alongside Windows so that you can continue using XP software.

Previously known as Cinnarch, Antergos is based on Arch Linux, which is popular with hard-core open source users, but Antergos much easier for beginners to use than Arch. It comes with a graphical installer that allows the user to choose from among several interfaces, including some that look quite a bit like XP.

Like Antergos, Manjaro aims to be a more user-friendly version of Arch. It comes with desktop environments, software management applications and media codecs pre-installed so users can get right to work after installing it.

For those looking to replace Windows XP on a PC primarily used by kids, Edubuntu is an excellent choice. It's based on Ubuntu (and supported by Canonical, the company behind Ubuntu), so it's very user-friendly. Plus, it adds plenty of software tailored for use by schools or home users with children.

Forked from Mandrake (which was later renamed Mandriva), Mageia is a community-driven Linux distribution with a good reputation for being beginner-friendly. Because it's updated very frequently, it tends to include more recent versions of software packages, and it has excellent support for several different languages.

Like Mageia, OpenMandriva is a community-managed Linux distribution based on Mandrake/Mandriva. It attempts to be simple and straightforward enough for new users but also to offer the breadth and depth of capabilities demanded by advanced users.

Kubuntu's goal is to "make your PC friendly," and it's fairly easy for new Linux users to figure out. It combines Ubuntu and the KDE desktop and includes plenty of built-in software, like a web browser, an office suite, media apps and more.

Netrunner is based on Kubuntu, plus some interface modifications to make it even more user friendly and some extra codecs to make it easier to play media files. The project also offers a second version of the same OS based on Manjaro.

Kwheezy is based on Debian, which is popular with advanced Linux users, but it's designed to be more accessible for Linux newcomers. It comes "with all the applications, plugins, fonts and drivers that you need for daily use, and some more," and it uses the intuitive KDE desktop.